Microspheres and disks as optical resonators are currently under intensive investigation for applications in biochemical sensing. While microspheres made of glass feature a very high Q-factor (>106), lack of an appropriate approach to mass-producing and aligning microsphere resonators has hindered their acceptance as viable products. Microdisks or microrings based on semiconductor wafers, on the other hand, are relatively easy to fabricate in a large quantity. Their positions with respect to waveguides can be adjusted using lithographic technologies such as dry/wet etching and layer deposition. The Q-factors of these resonators, however, are typically below 104, due at least in part to the surface roughness and to material absorption.
Other approaches to forming microcavities include forming cylindrical cavities by slicing through an optical fiber. This allows for mass-production of ring resonators with higher attainable Q-factors and controlled dimensions at low cost. However, such cylindrical microresonators feature only two-dimensional light confinement, and the light can propagate freely along the direction perpendicular to the planar surface. Consequently, any misalignment between the waveguide and the cylindrical ring resonator leads to the light being coupled into the lossy modes of the microcavity, resulting in degradation in the light enhancement.